We have tried to stop the electromagnetic emissions of the terminal of an optical HDMI cable with several layers of aluminum and copper tape, but the result was not enough.
For the side of the cable that is near the desk we have found that a thick aluminum box with no apertures can stop this radiation quite well. The little box must have no apertures at all, so the cables must pass through very small holes or over small dents on the box edge.
For the side of the cable that is attached to the computer the aluminum box is not possible because the small device has to be attached to an HDMI port of the computer. In this case maybe it is better to use a short HDMI cable extension from the computer to the metal box, and then put the optical HDMI terminal inside the metal box. Another possible solution is to enclose the entire computer with all the terminals attached to it, in a large metal enclosure with no apertures, with just a small hole on the back for all the cables.